polyester film

Abstract

To provide a new polyester film as a polyester film for a capacitor which has good heat resistance while maintaining electric characteristics equivalent to a conventional film, and can correspond to the conversion into the thin film.SOLUTION: A polyester film contains a polyester resin (X) and a polypropylene resin (Y), wherein a film cross section structure has a layered structure, and has a polypropylene resin layer (Yt) having a ratio (y / x) of a length (y) in a width direction (TD) to a length (x) in a thickness direction is 17 or more.

Description

Technical Field

[0001] The present invention relates to a polyester film, and particularly to a polyester film suitable for use as a capacitor.

Background Art

[0002] In various fields such as industrial materials, optical materials, electronic component materials, and battery packaging materials, polyethylene terephthalate (PET) films, which are typical polyester films, particularly biaxially stretched PET films, are widely used because of their excellent transparency, mechanical strength, heat resistance, flexibility, and the like.

[0003] In addition, due to excellent electrical properties such as low dielectric loss characteristics and high moisture resistance, resin films such as biaxially stretched polypropylene films are used as dielectric films for capacitors such as filter capacitors and smoothing capacitors for high-voltage capacitors, various switching power supplies, converters, and inverters. In recent years, there has been a need for further miniaturization and high capacity of capacitors. For example, when using a resin film for a capacitor for an inverter power supply device that controls a drive motor of an electric vehicle or a hybrid vehicle, small size, light weight, and high capacity are required (see Patent Document 1). With the increase in the capacitance of capacitors, for example, in a temperature range exceeding 120°C, there is a tendency for high withstand voltage characteristics (stability of capacitance) over a long period of time to be required.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] However, for example, with general-purpose biaxially oriented polypropylene films, while the electrical properties are good, the heat resistance of the film itself reaches its limit when the operating temperature range is above 120°C, making it difficult to use in such conditions.

[0006] Therefore, the present invention has been made in view of the above problems, and aims to propose a polyester film that is particularly suitable for use in capacitors, which maintains electrical properties comparable to conventional films, has good heat resistance, and can be made into a thin film. [Means for solving the problem]

[0007] As a result of diligent research, the inventors have found that the above problems can be solved by having a polypropylene resin layer (Yt) containing a polyester resin (X) and a polypropylene resin (Y), wherein the film cross-sectional structure has a layered structure, and the ratio (y / x) of the length in the thickness direction (x) to the length in the width direction (TD) (y) is 17 or more, and have completed the present invention as described below. In other words, the present invention provides the following [1] to

[19] . [1] A polyester film comprising a polyester resin (X) and a polypropylene resin (Y), wherein the cross-sectional structure of the film has a layered structure, and the polypropylene resin layer (Yt) has a ratio (y / x) of the length in the thickness direction (x) to the length in the width direction (TD) (y) of 17 or more. [2] The polypropylene resin layer (Yt) has 0.08 particles / μm 2 The polyester film described above [1] is as described above. [3] The polyester film according to [1] or [2] above, wherein the value obtained by dividing the ratio (y / x) by the film thickness (t (unit: μm)) is 1.5 or more. [4] A polyester film according to any one of [1] to [3] above, wherein the polypropylene resin layer (Yt) has a continuous structure from edge to edge in the width direction (TD) of an observation field of view of 19 μm × 25 μm. [5] A polyester film according to any one of [1] to [4] above, having a plurality of polypropylene resin layers (Yt), wherein a polyester resin layer (Xt) is interposed between adjacent polypropylene resin layers (Yt) in the thickness direction. [6] A polyester film comprising a polyester resin (X) and a polypropylene resin (Y), wherein the apparent occupancy rate of the polypropylene resin (Y) in the film by FIB (focused ion beam)-SEM is 10% or more. [7] A polyester film according to any of [1] to [6] above, comprising 1 to 30 parts by mass of polypropylene resin (Y) per 100 parts by mass of polyester resin (X). [8] A polyester film according to any one of [1] to [7] above, comprising 0.01 to 40 parts by mass of a compatibilizer (Z) per 100 parts by mass of polyester resin (X). [9] The polyester film according to [8] above, wherein the compatibilizer (Z) is a resin or ionomer having an acid anhydride structure [-C(=O)-OC(=O)-].

[10] The polyester film according to any one of [1] to [9] above, wherein the polycondensation catalyst of the polyester resin (X) is Ti-based.

[11] The polyester film according to any one of [1] to

[10] above, wherein the polyester resin (X) is at least one selected from polyethylene terephthalate and polyethylene-2,6-naphthalate.

[12] The polyester film according to any one of [1] to

[11] above, wherein a cured resin layer is provided on at least one surface of the polyester film.

[13] A polyester film according to any of [1] to

[12] above, having a film thickness of 0.5 to 12.0 μm.

[14] A polyester film according to any of the above [1] to

[13] , wherein the dielectric loss tangent (tanδ) at 1 kHz is 0.54 or less.

[15] A metal laminated film having a metal layer provided on at least one side of the polyester film described in any of [1] to

[14] above.

[16] A polyester film for use in capacitors, as described in any of [1] to

[14] above.

[17] A metal laminated film as described in

[15] above, for use with capacitors.

[18] The polyester film described in

[16] above, for use in condensers mounted on automobiles.

[19] The metal laminated film described in

[17] above, for use in a capacitor mounted in an automobile. [Effects of the Invention]

[0008] According to the present invention, it is possible to provide a polyester film for capacitors that maintains electrical properties comparable to conventional films, while also having good heat resistance, possessing heat resistance that can be used even in temperature ranges of 120°C or higher, and that can be made into a thin film. [Brief explanation of the drawing]

[0009] [Figure 1] This is a cross-sectional photograph (magnification: 5000x) of the polyester film (sample film) obtained in Example 1. [Figure 2] This is a cross-sectional photograph (magnification: 5000x) of the polyester film (sample film) obtained in Example 2. [Figure 3] This is a cross-sectional photograph (magnification: 5000x) of the polyester film (sample film) obtained in Example 3. [Figure 4] This is an XY plane image of the polyester film (sample film) obtained in Example 1. [Figure 5] This is an XY plane image of the polyester film (sample film) obtained in Example 2. [Figure 6] This is an XY plane image of the polyester film (sample film) obtained in Example 3. [Figure 7] This is a conceptual diagram showing the X, Y, and Z axes in FIB (Focused Ion Beam)-SEM measurements.

Mode for Carrying Out the Invention

[0010] Next, an example of an embodiment of the present invention will be described. However, the present invention is not limited to the embodiments described below.

[0011] [Polyester Film] The polyester film of the present invention (hereinafter sometimes referred to as "the present polyester film") contains a polyester resin (X) and a polypropylene resin (Y), has a film cross-sectional structure having a layered structure, and has a ratio (y / x) of the length (x) in the thickness direction to the length (y) in the width direction (TD) of 17 or more, and has a polypropylene resin layer (Yt).

[0012] The present polyester film has excellent physical properties such as heat resistance, flatness, optical properties, and strength. The above polyester film may be a single layer or a multilayer film (i.e., a laminated film) having two or more layers with different properties. Further, the present polyester film may be an unstretched film (sheet) or a stretched film. Among them, a stretched film stretched in a uniaxial direction or a biaxial direction is preferable. Among them, from the viewpoints of the balance of mechanical properties and flatness, a biaxially stretched film is more preferable. Therefore, a biaxially stretched polyester film is even more preferable.

[0013] In the case of a multilayer film, any one of the layers may be the above-described polyester film of the present invention (i.e., a layer having a polypropylene resin layer (Yt) and having a layered structure), and it is preferable that all layers are the present polyester film. For example, in the case of a three-layer structure multilayer film of surface layer / middle layer / surface layer, any one of the layers may be the present polyester film, but it is preferable that the middle layer is the present polyester film, and it is more preferable that all layers are the present polyester film. Therefore, in the case of a multilayer, any one of the layers may contain the polypropylene resin (Y) described later, and the layer containing the polypropylene resin (Y) preferably contains a compatibilizer (Z) as appropriate.

[0014] <Polyester resin (X)> The polyester resin (X), which is the main component resin of this polyester film, may be either homopolyester or copolymerized polyester. The main component resin refers to the resin with the largest mass percentage among the resins constituting the polyester film, and it is sufficient if it accounts for 50% or more by mass, or 75% or more by mass, or 90% or more by mass, or 100% by mass of the resins constituting the polyester film.

[0015] The homopolyester described above is preferably one obtained by polycondensation of an aromatic dicarboxylic acid and an aliphatic glycol. Examples of aromatic dicarboxylic acids include terephthalic acid and 2,6-naphthalenedicarboxylic acid, and examples of aliphatic glycols include ethylene glycol, diethylene glycol, 1,4-butanediol, and 1,4-cyclohexanedimethanol. Typical homopolyesters include polyethylene terephthalate (PET), polybutylene terephthalate (PBT), and polyethylene-2,6-naphthalate (PEN). In the present invention, polyethylene terephthalate (PET) and polyethylene-2,6-naphthalate (PEN) are particularly preferred, and these can also be used in combination.

[0016] On the other hand, if the polyester is a copolymerized polyester, it is preferable that it is a copolymer containing 30 mol% or less of a third component. Examples of dicarboxylic acid components in copolymerized polyesters include one or more types of isophthalic acid, phthalic acid, terephthalic acid, 2,6-naphthalenedicarboxylic acid, adipic acid, sebacic acid, etc., and examples of glycol components include one or more types of ethylene glycol, diethylene glycol, propylene glycol, 1,4-butanediol, 1,4-cyclohexanedimethanol, neopentyl glycol, etc. In particular, the polyester film is preferably polyethylene terephthalate in which 60 mol% or more, preferably 80 mol% or more, are ethylene terephthalate units, or polyethylene 2,6-naphthalate in which 60 mol% or more, preferably 80 mol% or more, are ethylene-2,6-naphthalate units.

[0017] (Polyester polycondensation catalyst) Examples of polycondensation catalysts used when obtaining the above-mentioned polyester by polycondensation include antimony compounds, germanium compounds, aluminum compounds, and titanium compounds. Among these, at least one of antimony compounds and titanium compounds is preferred, and in particular, polyester obtained using a titanium compound is preferred. Therefore, the polyester film preferably contains at least one of an antimony compound and a titanium compound, and more preferably contains a titanium compound (Ti-based). By using the aforementioned titanium compound, the number of metal-containing aggregates, so-called coarse foreign matter, derived from the titanium compound can be reduced in the film.

[0018] The polyester constituting the outermost layer of this film (also called the "surface layer," for example, the surface layer on which the cured resin layer described later is laminated) preferably uses a titanium compound as its polycondensation catalyst; for example, it is preferable that the surface layer contains a titanium compound. The outermost layer, in the case of a laminated film, is the outermost layer among multiple layers, and in the case of a single layer, it is the surface layer of that layer. The titanium element content derived from the titanium compound in the outermost layer is preferably 3 ppm by mass or more and 40 ppm by mass or less, and more preferably 4 ppm by mass or more and 35 ppm by mass or less. Within the above range, catalyst-induced impurities can be reduced without decreasing the manufacturing efficiency of polyester. Furthermore, from a similar viewpoint, it is preferable that the antimony compound content in the outermost layer of this film be 100 ppm by mass or less.

[0019] (particle) In the polyester resin (X), particles may be added primarily for the purpose of providing slipperiness and preventing scratches during each process. When particles are added, the type of particles is not particularly limited as long as they can provide slipperiness. Specific examples include inorganic particles such as silica, calcium carbonate, magnesium carbonate, barium carbonate, calcium sulfate, calcium phosphate, magnesium phosphate, kaolin, aluminum oxide, and titanium oxide, and organic particles such as acrylic resin, styrene resin, urea resin, phenolic resin, epoxy resin, and benzoguanamine resin. Furthermore, precipitated particles obtained by precipitating and finely dispersing a portion of metal compounds such as catalysts during the polyester manufacturing process can also be used.

[0020] On the other hand, there are no particular restrictions on the shape of the particles used; spherical, lumpy, rod-shaped, flattened, etc., may be used. Furthermore, there are no particular restrictions on their hardness, specific gravity, color, etc. Two or more types of these particles may be used in combination as needed. Furthermore, the average particle size of the particles used is preferably 3 μm or less, more preferably 0.1 to 2 μm, and particularly preferably in the range of 0.1 to 1 μm. By using particles with an average particle size within the above range, the polyester film can be given an appropriate surface roughness, ensuring good slipperiness and smoothness. When incorporating particles, for example, a surface layer and an intermediate layer can be provided, with the particles contained in the surface layer. In this case, a multilayer structure can be formed having a particle-containing surface layer, an intermediate layer, and a particle-containing surface layer in that order.

[0021] Furthermore, an embodiment in which the polyester resin (X) substantially does not contain particles is also preferred. Here, "substantially free of particles" means intentionally not containing particles, and specifically refers to a particle content (particle concentration) of 200 ppm by mass or less, more preferably 150 ppm by mass or less. The polyester film of the present invention contains a polyester resin (X) that constitutes the polyester and an immiscible polypropylene resin (Y). Therefore, the polypropylene resin (Y) can form fine irregularities on the film surface. As a result, the amount of particles in the film can be kept below the above upper limit, making it easier to impart slipperiness to the film while ensuring its transparency. Furthermore, if the polyester film contains no particles, or only a small amount of particles, the transparency of the base film will be high, resulting in a polyester film with a good appearance, but the slipperiness may be insufficient. In such cases, it is advisable to improve the slipperiness by incorporating particles into the cured resin layer, as described later.

[0022] <Polypropylene resin (Y)> The polypropylene resin (Y) may be a homopolymer of propylene, or a copolymer of propylene and ethylene and / or an α-olefin having 4 to 20 carbon atoms. In the present invention, there may be one copolymer component, or two or more copolymer components may be used in combination as needed.

[0023] The method for producing polypropylene resin (Y) is not particularly limited, and conventionally known methods can be employed, such as polymerizing propylene and any comonomer with a Ziegler-Natta catalyst. This is also described in Section 2.3.1 (pages 20-57) of the "Polypropylene Handbook," edited by Edward P. Moore Jr., translated and supervised by Tetsuo Yasuda and Nobu Sakuma, Kogyo Chosakai (1998).

[0024] (MFR (Melt Flow Rate)) In the present invention, the MFR is preferably in the range of 0.1 to 10 g / 10 min, more preferably 0.5 to 8 g / 10 min, and even more preferably 1 to 8 g / 10 min. By satisfying the above range, good fluidity can be ensured during film forming. The MFR was measured in accordance with ISO 1133:1997.

[0025] (Melting point) The melting point of the polypropylene resin (Y) is preferably 150°C or higher, more preferably 160°C or higher. A melting point above the lower limit ensures the desired heat resistance. While there is no particular upper limit to the melting point, it is typically 170°C. In this invention, the melting point can be determined by differential scanning calorimetry (DSC). Specifically, it is defined as the endothermic peak temperature when the temperature is first raised from room temperature to 200°C to erase the thermal history, then the temperature is lowered to 40°C at a cooling rate of 10°C / min, and then measured again at a heating rate of 10°C / min.

[0026] (Containing amount) The amount of polypropylene resin (Y) blended with 100 parts by mass of polyester resin (X) is preferably 1 to 30 parts by mass. More preferably, it is 5 to 20 parts by mass, and among these, 5 to 15 parts by mass is particularly good. By satisfying the above range, the polyester film for capacitors can have good electrical properties. Furthermore, if this polyester film is a multilayer film, the above blending amounts refer to the blending amounts for the entire polyester film. Therefore, even if, for example, a multilayer polyester film has layers with different blending amounts of polypropylene resin, it refers to the blending amount of polypropylene resin for the entire laminated film. However, if only some layers contain polypropylene resin (Y), and only some of those layers contain polypropylene resin layer (Yt), and the layers have a layered structure, then the amount of polypropylene resin (Y) in those layers should be as described above, and the total amount does not necessarily have to be within the above range.

[0027] <Compatibilizer (Z)> The polyester film of the present invention preferably contains a compatibilizer (Z). The compatibilizer (Z) is preferably a resin or ionomer having an acid anhydride structure.

[0028] (Resin having an acid anhydride structure [-C(=O)-OC(=O)-]) As resins having an acid anhydride structure [-C(=O)-OC(=O)-], you can use either polyethylene or polypropylene polyolefin resins having an acid anhydride structure [-C(=O)-OC(=O)-], or polystyrene resins having an acid anhydride structure. In particular, modified polypropylene resins having an acid anhydride structure (carboxylic acid-modified polypropylene resins) are preferred. The modified polypropylene resin having an acid anhydride structure refers to a resin synthesized using a monomer having an acid anhydride structure in addition to propylene as a raw material. Furthermore, a modified polyethylene resin having an acid anhydride structure can be used together with, or in place of, the modified polypropylene resin having an acid anhydride structure.

[0029] As monomers having an acid anhydride structure, compounds having an acid anhydride structure and an ethylenically unsaturated bond are preferred. Specific examples include maleic anhydride, citraconic anhydride, aconitic anhydride, and itaconic anhydride, among which maleic anhydride is preferred because it provides good compatibility between the polyester resin (X) and the polypropylene resin (Y).

[0030] If the acid value of a resin having an acid anhydride structure [-C(=O)-OC(=O)-] is too low, the interlayer adhesion between the resins may not be sufficiently achieved. On the other hand, if the acid value is too high, the compatibility with the resin decreases, making it difficult to keep the maximum void diameter within a predetermined range. For this reason, the acid value is preferably 0.5 to 50, and more preferably 0.5 to 10. Furthermore, the MFR is preferably 2 to 20 g / 10 min, and more preferably 3 to 10 g / 10 min. The acid value will be the value measured in accordance with JIS K 0070.

[0031] Commercially available resins with an acid anhydride structure can also be used. For example, when using modified polypropylene resin, Admer (trade name, manufactured by Mitsui Chemicals, Inc.) and OREVAC (trade name, manufactured by Arkema, Inc.) can be used. When using modified polyethylene resin, Admer (trade name, manufactured by Mitsui Chemicals, Inc.) LF128 (MFR2.7) can be used. Furthermore, when using styrene-based thermoplastic resins modified with maleic anhydride, examples include "Toughprene 912" from Asahi Kasei Corporation, "FG1901" and "FG1924" from Kraton Polymer Japan Co., Ltd., and "ToughTec M1911," "ToughTec M1913," and "ToughTec M1943" from Asahi Kasei Corporation.

[0032] (Ionomer) Ionomers are formed by using the cohesive force of metal ions to aggregate random, block, or graft copolymers of ethylene and acidic vinyl monomers such as unsaturated carboxylic acids, by partially neutralizing these copolymer chains with metal salts. Examples include ionomer resins obtained by neutralizing at least some of the carboxyl groups of an ethylene-unsaturated carboxylic acid copolymer resin with metal ions.

[0033] A copolymer resin containing ethylene-unsaturated carboxylic acid copolymer resin in which 10 mol% or more, preferably 10 to 90 mol%, and more preferably 15 to 80 mol%, of the carboxyl groups is neutralized with metal ions is preferably used. Examples of such metal ions include alkali metals such as lithium and sodium, polyvalent metal ions such as zinc, or alkaline earth metals such as magnesium and calcium. Among these, polyvalent metal ions such as zinc are preferred because they exhibit good compatibility.

[0034] (Containing amount) The amount of compatibilizer (Z) blended with 100 parts by mass of polyester resin (X) is preferably 0.01 to 40 parts by mass. More preferably, it is 0.1 to 20 parts by mass, and among these, 1 to 20 parts by mass is particularly preferred. By satisfying the above range, the compatibility between the polyester resin (X) and the polypropylene resin (Y) is improved, and a polyester film with good electrical properties can be obtained for use as a polyester film for capacitors. Furthermore, if this polyester film is a multilayer film, the above blending amounts refer to the blending amounts for the entire polyester film. Therefore, even in the case of a multilayer polyester film having layers with different blending amounts of the compatibilizer, for example, it refers to the blending amount of the compatibilizer for the entire laminated film. However, if only a portion of the layers contains polypropylene resin (Y), and only a portion of those layers has a layered structure, it is preferable that the portion of the layers contains a compatibilizer (Z). The amount of compatibilizer (Z) in that layer should be as described above, and the total amount does not necessarily have to be within the above range.

[0035] <Cured resin layer> The polyester film of the present invention preferably has a cured resin layer on at least one surface. The cured resin layer may be provided on at least one side of the polyester film, or it may be provided on both sides. The cured resin layer preferably contains a fluorine-containing compound. By providing a cured resin layer containing a fluorine-containing compound, the electrostatic potential per unit thickness is improved compared to polyester film alone, the attenuation of the electrostatic potential is further suppressed, and dust and oil adhering to the surface of the polyester film can be easily removed. In particular, the cured resin layer is preferably formed by curing a cured resin layer composition containing a fluorine-containing compound (A), a crosslinking agent (B), and a binder resin (C). By using a cured resin layer composition containing these components (A) to (C), it is possible to improve the electrostatic potential per unit thickness, further suppress the attenuation of the electrostatic potential, prevent scratches on the cured resin layer, improve solvent resistance, and improve the film-forming properties and transparency of the cured resin layer.

[0036] (Fluorine-containing compound (A)) The fluorine-containing compound is preferably a fluorine-containing resin from the viewpoint of increasing the strength of the cured resin layer. Specific examples of fluorine-containing resins include fluoroolefin copolymer resins using vinyl fluoride, vinylidene fluoride, tetrafluoroethylene, hexafluoropropylene, chlorotrifluoroethylene, etc. as monomers; fluorine copolymer resins obtained by polymerizing polyalkylene ethers in which some or all of the hydrogen atoms are substituted with fluorine atoms, such as fluoromethylene ether, difluoromethylene ether, fluoroethylene ether, difluoroethylene ether, tetrafluoroethylene ether, and hexafluoropropylene ether, with other monomers; fluorine copolymer resins obtained by graft polymerization of a hydroxyl group-containing fluorine resin copolymer with a (meth)acrylic acid ester compound or other monomers; and vinyl polymers having perfluoroalkyl groups. Among these, fluoroolefin copolymer resins and fluorine resin copolymers containing polyalkylene ether groups in which some or all of the hydrogen atoms are fluorinated are preferred from the viewpoint of excellent improvement of electrostatic potential and excellent wiping ability of dust and oil.

[0037] Among monomers of fluoroolefin copolymer resins, vinylidene fluoride, tetrafluoroethylene, and hexafluoropropylene are preferred. Furthermore, among polyalkylene ethers in which some or all hydrogen atoms are substituted with fluorine atoms, difluoromethylene ether, difluoroethylene ether, and tetrafluoroethylene ether are preferred. When the fluorine-containing resin consists of fluorine-containing monomers, it is preferable that it be a mixed dispersion with other components from the viewpoint of dispersibility in the solvent and compatibility with other resins. Other components will be described later.

[0038] Examples of fluorine-based copolymer resins obtained by polymerizing a polyalkylene ether in which some or all of the hydrogen atoms are replaced with fluorine atoms with other monomers include urethane resins having polyfluoroalkylene ether groups, polyester resins having polyfluoroalkylene ether groups, and acrylic resins having polyfluoroalkylene ether groups. Other monomers that constitute urethane resins having polyfluoroalkylene ether groups include isocyanate compounds. Examples of isocyanate compounds include aromatic diisocyanates such as tolylene diisocyanate, xylylene diisocyanate, methylenediphenyl diisocyanate, phenylene diisocyanate, naphthalene diisocyanate, and tolidine diisocyanate; aliphatic diisocyanates having aromatic rings such as α,α,α',α'-tetramethylxylylene diisocyanate; aliphatic diisocyanates such as methylene diisocyanate, propylene diisocyanate, lysine diisocyanate, trimethylhexamethylene diisocyanate, and hexamethylene diisocyanate; and alicyclic diisocyanates such as cyclohexane diisocyanate, methylcyclohexane diisocyanate, isophorone diisocyanate, dicyclohexylmethane diisocyanate, and isopropylidene dicyclohexyl diisocyanate.

[0039] Other monomers that make up the urethane resin include polyols that do not contain fluorine atoms, polyols having carboxyl groups such as dimethylolpropanoic acid, dimethylolbutanoic acid, bis-(2-hydroxyethyl)propionic acid, and bis-(2-hydroxyethyl)butanoic acid. Among these, it is preferable to include polyols having carboxyl groups from the viewpoint that they can self-emulsify when water is used as the dispersion solvent, and dimethylolpropanoic acid is more preferable among these.

[0040] The cured resin layer is preferably formed from a cured resin layer forming composition containing a fluorine-containing compound. The cured resin layer forming composition (hereinafter referred to as "cured resin layer composition") may consist of a fluorine-containing compound as a non-volatile component, or it may contain other components. The content of fluorine-containing compounds in the cured resin layer composition is preferably in the range of 5 to 100% by mass, more preferably 20 to 98% by mass, and even more preferably 45 to 95% by mass, relative to the non-volatile components in the cured resin layer composition. By setting the content to 5% by mass or more, it is possible to improve the electrostatic potential per unit thickness, further suppress the decay of the electrostatic potential, and make it possible to easily remove dust and oil adhering to the surface of the polyester film.

[0041] (Crosslinking agent (B)) The cured resin layer composition for forming the cured resin layer preferably contains a crosslinking agent (B) as described above. By including a crosslinking agent in the cured resin layer composition, a dense cured resin layer with a high crosslink density can be formed. Furthermore, scratches on the cured resin layer can be prevented, and solvent resistance can be improved. There are no particular restrictions on the crosslinking agent, and conventionally known crosslinking agents can be used. Examples of crosslinking agents include melamine compounds, oxazoline compounds, epoxy compounds, isocyanate compounds, carbodiimide compounds, and silane coupling compounds. The crosslinking agent is preferably at least one selected from melamine compounds, oxazoline compounds, and isocyanate compounds, and is preferably a melamine compound from the viewpoint of curability. These crosslinking agents may be used individually or in combination of two or more. Furthermore, any polymerizable monomer may be included in the cured resin layer composition as a component that cures together with these crosslinking agents.

[0042] (Melamine compound) The melamine compound used as a crosslinking agent is a compound having a melamine skeleton in the compound. For example, alkylolated melamine derivatives, compounds partially or completely etherified by reacting alkylolated melamine derivatives with alcohol, and mixtures thereof can be used. Examples of alkylolation include methylolation, ethylolation, isopropylolation, n-butyrolation, and isobutyrolation. Among these, methylolation is preferred from the viewpoint of reactivity. Suitable alcohols used for etherification include methyl alcohol, ethyl alcohol, isopropyl alcohol, n-butanol, and isobutanol. From the viewpoint of improving the coating strength of the cured resin layer and improving the adhesion between the cured resin layer and the polyester film, partially or completely etherified alkylolated melamine derivatives are preferred, and alkylol etherified with methyl alcohol is more preferred. The amount of partially etherified alkylol groups is preferably 0.5 to 5 equivalents, and more preferably 0.7 to 5 equivalents, relative to the unetherified alkylol groups. Furthermore, the melamine compound may be a monomer, a polymer of two or more commensals, or a mixture thereof. In addition, a compound in which urea or the like is co-condensed with a portion of the melamine may be used.

[0043] To increase the reactivity of the melamine compound, the cured resin layer composition may contain a crosslinking catalyst in addition to the melamine compound. Various known catalysts can be used as the crosslinking catalyst, for example, amine compounds, salts of amine compounds, organic acids such as aromatic sulfonic acid compounds like p-toluenesulfonic acid and phosphoric acid compounds and their salts, imine compounds, amidine compounds, guanidine compounds, organometallic compounds, and metal salts such as zinc stearate, zinc myristate, aluminum stearate, and calcium stearate. Among these, amine compounds, salts of amine compounds, and p-toluenesulfonic acid are preferred, and amine compounds and salts of amine compounds are more preferred.

[0044] (Oxazoline compounds) Oxazoline compounds are compounds having an oxazoline group in their molecule, and polymers containing an oxazoline group are particularly preferred. These polymers can be produced by polymerization of an addition-polymerizable oxazoline group-containing monomer alone or with other monomers. Among these, acrylic polymers, which are copolymers of an addition-polymerizable oxazoline group-containing monomer and an acrylic monomer having a (meth)acryloyl group, are preferred, and the acrylic polymer may have a polyalkylene oxide chain. In this specification, the term (meth)acryloyl group is used to mean either or both of the "acryloyl group" and the "methacryloyl group," and the same applies to other similar terms. Examples of addition-polymerizable oxazoline group-containing monomers include 2-vinyl-2-oxazoline, 2-vinyl-4-methyl-2-oxazoline, 2-vinyl-5-methyl-2-oxazoline, 2-isopropenyl-2-oxazoline, 2-isopropenyl-4-methyl-2-oxazoline, and 2-isopropenyl-5-ethyl-2-oxazoline. One or more of these can be used. Among these, 2-isopropenyl-2-oxazoline is preferred because it is readily available industrially. Addition-polymerizable oxazoline group-containing monomers may be used individually or in combination of two or more monomers.

[0045] Other monomers are not limited to monomers copolymerizable with addition polymerizable oxazoline group-containing monomers, such as (meth)acrylic acid esters such as alkyl (meth)acrylates (alkyl groups include methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, t-butyl, 2-ethylhexyl, and cyclohexyl groups); unsaturated carboxylic acids such as acrylic acid, methacrylic acid, itaconic acid, maleic acid, fumaric acid, crotonic acid, styrene sulfonic acid and their salts (salts include sodium salts, potassium salts, ammonium salts, and tertiary amine salts); unsaturated nitriles such as acrylonitrile and methacrylonitrile; (meth) Examples include unsaturated amides such as acrylamide, N-alkyl(meth)acrylamide, N,N-dialkyl(meth)acrylamide (alkyl groups include methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, t-butyl, 2-ethylhexyl, and cyclohexyl groups); vinyl esters such as vinyl acetate and vinyl propionate; vinyl ethers such as methyl vinyl ether and ethyl vinyl ether; α-olefins such as ethylene and propylene; halogen-containing α,β-unsaturated monomers such as vinyl chloride, vinylidene chloride, and vinyl fluoride; and α,β-unsaturated aromatic monomers such as styrene and α-methylstyrene.

[0046] Furthermore, monomers having polyalkylene oxide chains can also be used as other monomers. Examples of monomers having polyalkylene oxide chains include esters obtained by adding polyalkylene oxide to the carboxyl group of unsaturated carboxylic acids such as acrylic acid and methacrylic acid. Examples of polyalkylene oxide chains include polymethylene oxide, polyethylene oxide, polypropylene oxide, and polybutylene oxide. The repeating units of the polyalkylene oxide chain are preferably in the range of 3 to 100. Other monomers used in oxazoline compounds may be used individually or in combination of two or more.

[0047] The amount of oxazoline groups in the oxazoline compound is preferably in the range of 0.5 to 10 mmol / g, more preferably 1 to 9 mmol / g, even more preferably 3 to 8 mmol / g, and particularly preferably 4 to 6 mmol / g. Using the compound within this range tends to improve the durability of the coating film (cured resin layer).

[0048] (Epoxy compound) Epoxy compounds are compounds that have an epoxy group in their molecule. Examples include condensates of epichlorohydrin with hydroxyl or amino group-containing compounds such as ethylene glycol, polyethylene glycol, glycerin, polyglycerin, and bisphenol A. Examples of epoxy compounds include polyepoxy compounds, diepoxy compounds, monoepoxy compounds, and glycidylamine compounds. Examples of polyepoxy compounds include sorbitol polyglycidyl ether, polyglycerol polyglycidyl ether, pentaerythritol polyglycidyl ether, diglycerol polyglycidyl ether, triglycidyl tris(2-hydroxyethyl) isocyanate, glycerol polyglycidyl ether, and trimethylolpropane polyglycidyl ether. Examples of diepoxy compounds include neopentyl glycol diglycidyl ether, 1,6-hexanediol diglycidyl ether, resorcinol diglycidyl ether, ethylene glycol diglycidyl ether, polyethylene glycol diglycidyl ether, propylene glycol diglycidyl ether, polypropylene glycol diglycidyl ether, and polytetramethylene glycol diglycidyl ether. Examples of monoepoxy compounds include allyl glycidyl ether, 2-ethylhexyl glycidyl ether, and phenyl glycidyl ether. Examples of glycidylamine compounds include N,N,N',N'-tetraglycidyl-m-xylylenediamine and 1,3-bis(N,N-diglycidylamino)cyclohexane. The epoxy compound may be used alone or in combination of two or more types.

[0049] (Isocyanate compounds) Isocyanate compounds are compounds having an isocyanate derivative structure, such as isocyanates or blocked isocyanates. Examples of isocyanates include aromatic isocyanates such as tolylene diisocyanate, xylylene diisocyanate, methylenediphenyl diisocyanate, phenylene diisocyanate, and naphthalene diisocyanate; aliphatic isocyanates having an aromatic ring such as α,α,α',α'-tetramethylxylylene diisocyanate; aliphatic isocyanates such as methylene diisocyanate, propylene diisocyanate, lysine diisocyanate, trimethylhexamethylene diisocyanate, and hexamethylene diisocyanate; and alicyclic isocyanates such as cyclohexane diisocyanate, methylcyclohexane diisocyanate, isophorone diisocyanate, methylenebis(4-cyclohexyl isocyanate), and isopropylidene dicyclohexyl diisocyanate. Polymers and derivatives of these isocyanates, such as biuretized, isocyanurateized, and uretdioneized compounds, are also examples. These isocyanates may be used individually or in combination of multiple types. Among the above isocyanates, aliphatic isocyanates or alicyclic isocyanates are more preferred in order to avoid yellowing due to ultraviolet light.

[0050] When used in the form of blocked isocyanates, examples of blocking agents include bisulfites such as sodium bisulfite, phenolic compounds such as phenol, cresol, and ethylphenol; alcoholic compounds such as propylene glycol monomethyl ether, ethylene glycol, benzyl alcohol, methanol, and ethanol; active methylene compounds such as dimethyl malonate, diethyl malonate, methyl acetoacetate, ethyl acetoacetate, and acetylacetone; mercaptan compounds such as butyl mercaptan and dodecyl mercaptan; lactam compounds such as ε-caprolactam and δ-valerolactam; amine compounds such as diphenylaniline, aniline, and ethyleneimine; acid amide compounds such as acetanilide and acetic acid amide; and oxime compounds such as formaldehyde, acetaldehyde oxime, acetone oxime, methyl ethyl ketone oxime, and cyclohexanone oxime. These may be used individually or in combination of two or more.

[0051] Furthermore, isocyanate compounds may be used individually or as compounds with various polymers. In addition, isocyanate compounds may be incorporated into the cured resin layer composition as a mixture of various polymers. From the viewpoint of improving the dispersibility and crosslinking properties of isocyanate compounds, it is preferable to use mixtures or compounds with polyester resin or urethane resin. One type of isocyanate compound may be used alone or two or more types may be used. Note that the amount of crosslinking agent when using isocyanate compounds includes the amount of the blocking agent and polymers that are bonded or mixed into the composition.

[0052] (Carbodiimide compounds) Carbodiimide compounds are compounds having a carbodiimide structure. Using carbodiimide compounds can improve the heat and humidity resistance of cured resin layers. Carbodiimide compounds can be synthesized using conventionally known techniques, and generally, a condensation reaction of diisocyanate compounds is employed. The diisocyanate compound is not particularly limited; both aromatic and aliphatic compounds can be used. Specifically, examples include tolylene diisocyanate, xylene diisocyanate, diphenylmethane diisocyanate, phenylene diisocyanate, naphthalene diisocyanate, hexamethylene diisocyanate, trimethylhexamethylene diisocyanate, cyclohexane diisocyanate, methylcyclohexane diisocyanate, isophorone diisocyanate, dicyclohexyl diisocyanate, and dicyclohexylmethane diisocyanate. Carbodiimide compounds may be used individually or in combination of two or more.

[0053] (Silane coupling compounds) Silane coupling compounds are organosilicon compounds that contain both an organic functional group and a hydrolysis group such as an alkoxy group within a single molecule. For example, epoxy group-containing compounds such as 3-glycidoxypropylmethyldimethoxysilane, 3-glycidoxypropyltrimethoxysilane, 3-glycidoxypropylmethyldiethoxysilane, 3-glycidoxypropyltriethoxysilane, and 2-(3,4-epoxycyclohexyl)ethyltrimethoxysilane; vinyl group-containing compounds such as vinyltrimethoxysilane and vinyltriethoxysilane; styryl group-containing compounds such as p-styryltrimethoxysilane and p-styryltriethoxysilane; (meth)acrylic group-containing compounds such as 3-(meth)acryloxypropyltrimethoxysilane, 3-(meth)acryloxypropyltriethoxysilane, 3-(meth)acryloxypropylmethyldimethoxysilane, and 3-(meth)acryloxypropylmethyldiethoxysilane; 3-aminopropyltrimethoxysilane, 3-aminopropyltriethoxysilane, N-2-(aminoethyl)- Examples include amino group-containing compounds such as 3-aminopropyltrimethoxysilane, N-2-(aminoethyl)-3-aminopropyltriethoxysilane, N-2-(aminoethyl)-3-aminopropylmethyldimethoxysilane, N-2-(aminoethyl)-3-aminopropylmethyldiethoxysilane, 3-triethoxysilyl-N-(1,3-dimethylbutylidene)propylamine, N-phenyl-3-aminopropyltrimethoxysilane, and N-phenyl-3-aminopropyltriethoxysilane; isocyanurate group-containing compounds such as tris(trimethoxysilylpropyl)isocyanurate and tris(triethoxysilylpropyl)isocyanurate; and mercapto group-containing compounds such as 3-mercaptopropyltrimethoxysilane, 3-mercaptopropyltriethoxysilane, 3-mercaptopropylmethyldimethoxysilane, and 3-mercaptopropylmethyldiethoxysilane. Among the above compounds, epoxy group-containing silane coupling agents, double bond-containing silane coupling agents such as vinyl groups and (meth)acrylic groups, and amino group-containing silane coupling agents are more preferred from the viewpoint of maintaining the strength of the cured resin layer. Silane coupling agents may be used individually or in combination of two or more types.

[0054] The crosslinking agents contained in the cured resin layer composition should be designed to react during the drying process and film formation process when forming the cured resin layer, thereby improving the performance of the cured resin layer. It can be inferred that unreacted crosslinking agents, reacted compounds, or mixtures thereof are present in the cured resin layer formed from the cured resin layer composition.

[0055] The crosslinking agent content in the cured resin layer composition is preferably in the range of 5 to 60% by mass relative to the non-volatile components in the cured resin layer composition. By setting the crosslinking agent content in the cured resin layer composition to 5 to 60% by mass relative to the non-volatile components, the attenuation of the electrostatic potential is more easily suppressed. In addition, the strength of the cured resin layer is improved, and scratch resistance is also easily improved. From the above viewpoint, it is more preferably 10 to 50% by mass, even more preferably 15 to 40% by mass, and particularly preferably 20 to 40% by mass.

[0056] Furthermore, when a crosslinking catalyst is included in the cured resin layer composition, a crosslinking catalyst content in the range of 0.4 to 10% by mass relative to the non-volatile components in the cured resin layer composition is preferable, as it tends to improve the strength of the cured resin layer and enhance scratch resistance. From this viewpoint, the crosslinking catalyst content is preferably 0.6 to 8% by mass, more preferably 0.8 to 5% by mass.

[0057] (Binder resin (C)) The binder resin is a polymer component contained in the cured resin layer composition other than the polymer formed by the crosslinking of the above-mentioned crosslinking agent (B). The inclusion of the binder resin in the cured resin layer composition improves the film-forming properties and transparency of the cured resin layer.

[0058] Specific examples of binder resins include acrylic resin, polyvinyl alcohol, polyester resin, urethane resin, polyalkylene glycol, polyalkyleneimine, methylcellulose, hydroxycellulose, and starches. Among these, acrylic resin, polyester resin, and urethane resin are preferred from the viewpoint of applicability, acrylic resin and polyester resin are more preferred from the viewpoint of improving the durability of the cured resin layer itself, and acrylic resin is even more preferred from the viewpoint of further improving applicability.

[0059] (Acrylic resin) Acrylic resins are polymers composed of polymerizable monomers, including acrylic and methacrylic monomers. These may be homopolymers, copolymers, or copolymers with polymerizable monomers other than acrylic and methacrylic monomers. Furthermore, copolymers of these polymers with other polymers (e.g., polyester, polyurethane, etc.) are also included. Examples include block copolymers and graft copolymers. In other words, the acrylic resin may be an acrylic-modified polyester resin or an acrylic-modified polyurethane resin. Furthermore, polymers (and possibly mixtures of polymers) obtained by polymerizing polymerizable monomers in a polyester solution or polyester dispersion are also included. Similarly, polymers (and possibly mixtures of polymers) obtained by polymerizing polymerizable monomers in a polyurethane solution or polyurethane dispersion are also included. In the same manner, polymers (and possibly polymer mixtures) obtained by polymerizing polymerizable monomers in other polymer solutions or dispersions are also included, and these are also referred to as acrylic-modified polyester resins and acrylic-modified polyurethane resins in this specification. The polyesters and polyurethanes used in acrylic resins can be appropriately selected from those exemplified as polyesters and polyurethanes used in binder resins, as described later. Furthermore, the acrylic resin may contain hydroxyl groups and amino groups to further improve its adhesion to the polyester film.

[0060] The polymerizable monomers mentioned above are not particularly limited, but some representative compounds include, for example, various carboxyl group-containing monomers such as acrylic acid, methacrylic acid, crotonic acid, itaconic acid, fumaric acid, maleic acid, and citraconic acid, and their salts; various hydroxyl group-containing monomers such as 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 4-hydroxybutyl (meth)acrylate, monobutyl hydroxyl fumarate, and monobutyl hydroxyitaconate; methyl (meth)acrylate, ethyl (meth)acrylate, propyl (meth)acrylate, butyl (meth)acrylate, and lauryl (meth)acrylate. Examples include various (meth)acrylic acid esters such as acrylates; various nitrogen-containing compounds such as (meth)acrylamide, diacetone acrylamide, N-methylolacrylamide, or (meth)acrylonitrile; various styrene derivatives such as styrene, α-methylstyrene, divinylbenzene, and vinyltoluene; various vinyl esters such as vinyl propionate; various silicon-containing polymerizable monomers such as γ-methacryloxypropyltrimethoxysilane and vinyltrimethoxysilane; phosphorus-containing vinyl monomers; various vinyl halides such as vinyl chloride and pyridene chloride; and various conjugated dienes such as butadiene.

[0061] (Polyester resin) Polyester resins primarily consist of polycarboxylic acids and polyhydroxy compounds, such as those listed below. Specifically, as polycarboxylic acids, terephthalic acid, isophthalic acid, orthophthalic acid, phthalic acid, 4,4'-diphenyldicarboxylic acid, 2,5-naphthalenedicarboxylic acid, 1,5-naphthalenedicarboxylic acid, 2,6-naphthalenedicarboxylic acid, 2,7-naphthalenedicarboxylic acid, 1,4-cyclohexanedicarboxylic acid, 2-potassium sulfoterephthalic acid, 5-sodium sulfisoisophthalic acid, adipic acid, azelaic acid, sebacic acid, dodecanedicarboxylic acid, glutaric acid, succinic acid, trimellitic acid, trimesic acid, pyromellitic acid, trimellitic anhydride, phthalic anhydride, p-hydroxybenzoic acid, monopotassium salt of trimellitic acid, and their ester-forming derivatives can be used. Examples of polyvalent hydroxy compounds that can be used include ethylene glycol, 1,2-propylene glycol, 1,3-propylene glycol, 1,3-propanediol, 1,4-butanediol, 1,6-hexanediol, 2-methyl-1,5-pentanediol, neopentyl glycol, 1,4-cyclohexanedimethylol, p-xylylene glycol, bisphenol A-ethylene glycol adduct, diethylene glycol, triethylene glycol, polyethylene glycol, polypropylene glycol, polytetramethylene glycol, polytetramethylene oxide glycol, dimethylolpropionic acid, glycerin, trimethylolpropane, sodium dimethylolethylsulfonate, and potassium dimethylolpropionate. From these compounds, one or more can be appropriately selected, and a polyester resin can be synthesized by a conventional polycondensation reaction. Furthermore, the polyester resin may also be in the form of an aqueous dispersion, in which case hydrophilic functional groups may be introduced into the polyester resin as appropriate.

[0062] (Polyvinyl alcohol) Polyvinyl alcohol refers to a compound having a polyvinyl alcohol moiety. Conventional known polyvinyl alcohols can be used, including modified compounds such as those partially acetalized or butyralized. The degree of polymerization of polyvinyl alcohol is not particularly limited, but is usually 100 or higher, preferably in the range of 300 to 40000. A degree of polymerization of 100 or higher prevents a decrease in the water resistance of the cured resin layer. Furthermore, while the degree of saponification of polyvinyl alcohol is not particularly limited, polyvinyl acetate saponified polyvinyl acetate with a degree of 70 mol% or higher, preferably in the range of 70 to 99.9 mol%, more preferably 80 to 97 mol%, and particularly preferably 86 to 95 mol%, is practically used.

[0063] (urethane resin) Urethane resin is a polymer compound that contains urethane bonds within its molecule. Typically, urethane resin is produced by the reaction of a polyol with an isocyanate. Examples of polyols include polycarbonate polyols, polyester polyols, polyether polyols, polyolefin polyols, and acrylic polyols. These compounds may be used individually or in combination. The urethane resin may also be in aqueous dispersion form; in this case, for example, hydrophilic functional groups may be introduced into the polyol as appropriate.

[0064] The binder resin content in the cured resin layer composition is preferably in the range of 10 to 70% by mass relative to the non-volatile components in the cured resin layer composition. By setting the binder resin content to 10 to 70% by mass, it becomes easier to suppress the decay of coatability and electrostatic potential. It also becomes easier to improve the appearance and transparency of the cured resin layer. From these viewpoints, it is more preferably 20 to 65% by mass, even more preferably 30 to 60% by mass, and particularly preferably 30 to 55% by mass. In addition to the components mentioned above, the cured resin layer composition may also contain additives such as reaction modifiers, adhesion enhancers, surfactants, antistatic agents, and particles as appropriate.

[0065] The cured resin layer composition is preferably applied to a polyester film as a liquid coating solution, dried and cured as necessary. It is preferable to dilute the cured resin layer composition with a solvent to make it a coating solution. Each of the above components constituting the cured resin composition (components (A) to (C), etc.) may be dissolved in the solvent or dispersed in the solvent. There are no restrictions on the solvent used in the cured resin layer composition; either water or an organic solvent may be used. However, from the viewpoint of environmental protection, it is preferable to use an aqueous coating solution with water as the solvent. The aqueous coating solution may contain a small amount of organic solvent. The specific amount of organic solvent should be less than the amount of water by mass, for example, less than 30% by mass, preferably less than 20% by mass, and more preferably less than 10% by mass of the solvent. Examples of organic solvents used in combination with water include alcohols such as ethanol, isopropanol, ethylene glycol, and glycerin; ethers such as ethyl cellosolve, t-butyl cellosolve, propylene glycol monomethyl ether, and tetrahydrofuran; ketones such as acetone and methyl ethyl ketone; esters such as ethyl acetate; and amines such as dimethylethanolamine. These can be used individually or in combination. By appropriately selecting and including these organic solvents in the aqueous coating solution as needed, the stability and applicability of the coating solution can be improved.

[0066] Furthermore, when using an organic solvent alone as the solvent, examples of organic solvents include aromatic hydrocarbons such as toluene; aliphatic hydrocarbons such as hexane, heptane, and isooctane; esters such as ethyl acetate and butyl acetate; ketones such as ethyl methyl ketone (MEK) and isobutyl methyl ketone; alcohols such as ethanol and 2-propanol; and ethers such as diisopropyl ether and dibutyl ether. These may be used individually or in combination, taking into consideration solubility, applicability, boiling point, etc.

[0067] <Method for forming a hardened resin layer> The method for forming the cured resin layer will be described in detail below. The cured resin layer may be formed by in-line coating or by offline coating. In-line coating is a method of applying a coating solution of the cured resin layer composition to the surface of a polyester film on the manufacturing line for producing polyester films. Offline coating is a method of applying the coating solution to a polyester film that has already been manufactured, outside of the manufacturing line. From the viewpoint of ease of processing, it is preferable to form the cured resin layer by in-line coating.

[0068] In-line coating is a method of applying a coating solution of a cured resin layer composition to a polyester film at any stage from melt extrusion of polyester to stretching, heat fixing, and winding. Typically, the coating solution is applied to the polyester film at one of the following stages: an unstretched sheet obtained by melting and rapid cooling, a stretched uniaxially oriented film, a biaxially oriented film before heat fixing, or a film after heat fixing but before winding.

[0069] Conventional coating methods such as air doctor coating, blade coating, rod coating, bar coating, knife coating, squeeze coating, impregnation coating, reverse roll coating, transfer roll coating, gravure coating, kiss roll coating, cast coating, spray coating, curtain coating, calendr coating, and extrusion coating can be used as methods for applying the coating solution.

[0070] Furthermore, although not particularly limited, in sequential biaxial stretching, for example, a method is preferred in which a coating solution is applied to a uniaxially stretched film that has been stretched in the longitudinal direction (vertical direction), and then stretched in the transverse direction. With this method, the formation of the polyester film and the cured resin layer can be performed simultaneously, which has advantages in terms of manufacturing costs. In addition, because stretching is performed after coating, the thickness of the cured resin layer can be changed according to the stretching ratio, making thin-film coating easier compared to offline coating. Moreover, the thickness of the cured resin layer can be made more uniform.

[0071] Furthermore, by applying the coating solution of the cured resin layer composition onto the polyester film before stretching, the cured resin can be stretched together with the polyester film, thereby firmly adhering the cured resin layer to the polyester film. Furthermore, in the manufacturing of biaxially oriented films, the polyester film can be restrained in both the longitudinal and transverse directions by gripping the film edges with clips or the like while stretching. This allows for heating to high temperatures during the heat-setting process while maintaining flatness and preventing wrinkles. As a result, the heat treatment applied after coating the cured resin layer composition can reach temperatures that cannot be achieved by other methods, thus enabling stronger adhesion between the cured resin layer and the polyester film.

[0072] Furthermore, the coating liquid of the cured resin layer composition applied to the polyester film may be subjected to either heat treatment or active energy ray irradiation such as ultraviolet irradiation, or both, in both cases, whether it is an offline coating or an in-line coating, but at least heat treatment is preferred. In addition, the cured resin layer composition may be cured by either or both heat treatment and active energy ray irradiation. The heat treatment may be performed by heating in a heat setting process as described above, for example, but it may also be performed by other methods. Furthermore, if the coating liquid of the cured resin layer composition contains a solvent, it should be dried as appropriate, but drying by the heat treatment described above is preferred. Furthermore, in order to improve the applicability of the coating solution for forming the cured resin layer to the polyester film and the adhesion of the cured resin layer to the polyester film, surface treatments such as chemical treatment, corona discharge treatment, plasma treatment, ozone treatment, chemical treatment, and solvent treatment may be applied to the surface of the polyester film on which the cured resin layer will be formed before applying the coating solution.

[0073] As described above, the polyester film with a cured resin layer may have other layers between the polyester film and the cured resin layer. Examples of such other layers include layers with various functions, such as an antistatic layer, an easy-adhesion layer, and an oligomer encapsulation layer.

[0074] The thickness of the cured resin layer is preferably 0.005 to 1 μm. A thickness of 1 μm or less suppresses the migration of components from the cured resin layer to the cured resin layer itself, and also makes it easier to prevent blocking during winding. On the other hand, a thickness of 0.005 μm or more makes it easier to suppress the attenuation of the electrostatic potential. From these viewpoints, the thickness of the cured resin layer is more preferably 0.01 μm or more, even more preferably 0.02 μm or more, even more preferably 0.2 μm or less, even more preferably 0.1 μm or less, and among these, 0.06 μm or less is particularly preferable.

[0075] <Metal laminated film> The polyester film of the present invention can be a metal laminated film having a metal layer provided on at least one side thereof. The metal layer may be provided on one side of the polyester film or on both sides. It may also be provided on the cured resin layer described above. Examples of metals include copper, silver, chromium, aluminum, nickel, and zinc. Of these, aluminum and zinc are preferred from the standpoint of cost and environmental considerations. The thickness of the metal layer is preferably in the range of 10 to 5000 Å, more preferably in the range of 100 to 4000 Å, and even more preferably in the range of 100 to 2000 Å. Being within the above range is advantageous in terms of electrical properties.

[0076] <Method for manufacturing this polyester film> As an example of a manufacturing method for this polyester film, we will describe a manufacturing method when the polyester film is a biaxially oriented film. However, this is not the only manufacturing method we can describe.

[0077] First, raw materials, such as polyester chips, are supplied to a melt extruder by known methods, heated to above the melting point of each polymer, the molten polymer is extruded from the die, and cooled and solidified on a rotating cooling drum to a temperature below the glass transition point of the polymer, thereby obtaining a substantially amorphous, unoriented sheet.

[0078] Next, the unoriented sheet is stretched in one direction using a roll or tenter type stretcher. At this time, the stretching temperature is usually 25 to 120°C, preferably 35 to 100°C, and the stretching ratio is usually 2.5 to 7 times, preferably 2.8 to 6 times. Next, the material is stretched in a direction perpendicular to the stretching direction of the first stage. At this stage, the stretching temperature is usually 50 to 140°C, and the stretching ratio is usually 3.0 to 7 times, preferably 3.5 to 6 times. Furthermore, in the extension described above, a method of extending in one direction in two or more stages can also be adopted. Furthermore, in the present invention, as described above, it is preferable to increase the stretching ratio in the lateral direction, and therefore, it is preferable to set the stretching ratio to 4.5 times or more in either the first or second stage.

[0079] After stretching, the polyester film can be obtained as a biaxially oriented film by continuing to perform a heat-fixing treatment at a temperature of 130 to 270°C under tension or under relaxation of 30% or less. This polyester film can have its heat resistance and other properties improved by heat setting treatment. The above explanation of the manufacturing method assumes a single-layer film. However, for multi-layer films, it is advisable to first produce an unoriented sheet by co-extrusion, for example, and then proceed in the same manner.

[0080] (Film thickness) The thickness of the polyester film of the present invention is preferably 0.5 to 12.0 μm. A thickness within this range is suitable for use in capacitor applications. From this viewpoint, the thickness of the polyester film is more preferably 0.5 to 10.0 μm, and among these, 1.0 to 8.0 μm is particularly preferable.

[0081] <Characteristics of this polyester film> (Cross-sectional structure of the film) The polyester film of the present invention has a layered cross-sectional structure and comprises a polypropylene resin layer (Yt) in which the ratio (y / x) of the length in the thickness direction (x) to the length in the width direction (TD) (y) is 17 or more. Preferably, in addition to the above, at least one of the following additional requirements 1) to 4) should be satisfied simultaneously. 1) The number of polypropylene resin layers (Yt) where the ratio (y / x) of the length in the thickness direction (x) to the length in the width direction (TD) (x) is 17 or greater is 0.08 layers / μm 2 The above is preferable. More preferably 0.16 particles / μm 2 The most preferred value is 0.20 particles / μm. 2 That's all. 2) The value obtained by dividing the ratio (y / x) by the film thickness (t) is preferably 1.5 or more, more preferably 2.0 or more, even more preferably 4.0 or more, and particularly good if it is 5.0 or more. 3) It is preferable that the polypropylene resin layer (Yt) has a continuous structure from edge to edge in the width direction (TD) of the observation field. The size of the observation field is 19 μm × 25 μm. 4) Preferably, the material has a layered structure in which there are multiple polypropylene resin layers (Yt), and polyester resin layers (Xt) are interposed between adjacent polypropylene resin layers (Yt) in the thickness direction. By satisfying at least one, preferably two or more, more preferably three or more, and most preferably all of the above conditions 1) to 4) simultaneously, it is possible to obtain a polyester film with good dielectric loss tangent.

[0082] Specific means for forming the above structure include, for example, a) taking sufficient mixing time at the raw material formulation stage, b) setting the stretching ratio, especially the transverse stretching ratio, to 4.5 times or more from the perspective of stretching conditions, c) selecting a type of polypropylene resin with an MFR of 0.1 to 10 g / 10 min or less, and d) using a compatibilizer in combination. These methods can be used individually or in combination.

[0083] In this invention, to accommodate capacitor applications, the film thickness differs from conventional methods, with ultra-thin regions (0.5 to 12 μm) being the mainstream. Therefore, it differs from other applications (for example, to provide cushioning in paper substitutes) where incompatible polypropylene resin is used with polyester resin to intentionally form voids. The reason for using polypropylene resin is that it is used as a film for capacitors, particularly to improve electrical characteristics in the low-frequency range. From this perspective, the design philosophy is that minimizing the formation of voids derived from the polypropylene resin in the film after film molding results in better electrical characteristics. As mentioned above, the present invention was completed by focusing on the relationship between the cross-sectional structure of the film and its electrical characteristics (dielectric loss tangent).

[0084] Examples 1-3, which used a compatibilizer with a specific structure, showed good compatibility with Comparative Example 1 despite having a higher polypropylene content, and the film cross-sectional structure was found to have a flattened layered structure. Moreover, the layered structure originated from the polypropylene resin (Y) and was observed in large numbers within the observation field of view. In relation to electrical properties (dielectric loss tangent), the presence of more layers in the layered structure in the film thickness direction tended to result in better electrical properties. Furthermore, although the mechanism for forming the aforementioned layered structure is unknown, it is presumed that when stretching a film with a thickness in the ultrathin region (0.5 to 12 μm), forces are applied from above and below in the thickness direction of the film. It is presumed that the forces applied from above and below in the thickness direction of the film, combined with a synergistic effect with the compatibilizer, improve the adhesion between the polyester resin layer (Xt) and the polypropylene resin layer (Yt). As a result, for example, by pushing out the air present at the interface between the two, the resulting structure is less likely to form voids.

[0085] (Apparent occupancy rate of polypropylene resin in the film (%)) It is preferable that the apparent occupancy rate of polypropylene resin (Y) in the film, as measured by FIB (focused ion beam)-SEM, be 10% or more, more preferably 20% or more, and particularly preferably 30% or more. This method aims to visualize a three-dimensional (3D) image by taking 200 consecutive 2D SEM images and stitching them together. As a result, it has the advantage of being able to visualize the dispersion of polypropylene resin (Y) in three dimensions, determining how well it is dispersed throughout the film. The inventors of the present invention conducted a verification based on the hypothesis that even if the cross-sectional structure (XZ plane) is at the same level, the dispersibility of the polypropylene resin (Y) differs in depth (XY plane), as shown in the conceptual diagram in Figure 7. They also found that satisfying the apparent occupancy rate of the polypropylene resin (Y) as described above further improves the electrical properties (dielectric loss tangent).

[0086] (Dielectric loss tangent (tanδ)) The polyester film of the present invention preferably has a tanδ of 0.54 or less at 1 kHz, more preferably 0.50 or less, even more preferably 0.45 or less, and among these, 0.40 or less is particularly preferable. When tanδ satisfies the above range, the electrical properties of the film become good, making it suitable for use in capacitors.

[0087] <Application> The polyester film and metal laminated film of the present invention are useful for capacitors because they possess excellent electrical properties, such as a low dielectric loss tangent (tanδ). In particular, because they are thin films, they are useful for capacitors installed in automobiles such as hybrid vehicles and electric vehicles, where miniaturization, weight reduction, and high capacitance are required.

[0088] <Explanation of terms, etc.> In this invention, the term "film" includes "sheets," and the term "sheet" includes "film." In this invention, when "X~Y" (where X and Y are any numbers) is written, unless otherwise specified, it means "X or greater and Y or less," and also includes the meaning of "preferably greater than X" or "preferably less than Y." Furthermore, when "X or greater" (where X is any number) is written, unless otherwise specified, it includes the meaning of "preferably greater than X," and when "Y or less" (where Y is any number) is written, unless otherwise specified, it also includes the meaning of "preferably less than Y." [Examples]

[0089] Next, the present invention will be described in more detail with reference to examples. However, the present invention is not limited to the examples described below.

[0090] <Evaluation Method> The methods for measuring and evaluating various physical properties and characteristics are as follows:

[0091] (1) Intrinsic viscosity (IV) 1 g of polyester was accurately weighed, dissolved in 100 ml of a phenol / tetrachloroethane mixed solvent (50 / 50 by mass ratio), and measured at 30°C.

[0092] (2) Film thickness After fixing and molding small film pieces with epoxy resin, they were cut with a microtome, and the cross-sections of the film were observed using transmission electron microscopy. Two interfaces, separated by light and dark areas, were observed running approximately parallel to the film surface. The distance between these two interfaces and the film surface was measured from 10 photographs, and the average value was defined as the film thickness.

[0093] (3) Melting point of polypropylene resin (Y) The temperature is defined as the endothermic peak top temperature when the temperature is raised from room temperature to 200°C using differential scanning calorimetry (DSC), the thermal history is erased, the temperature is then lowered to 40°C at a rate of 10°C / min, and then measured again at a heating rate of 10°C / min.

[0094] (4) The ratio (y / x) of the length (x) in the thickness direction and the length (y) in the width direction (TD) of the polypropylene resin layer (Yt). The cross-sectional structure of the sample film was observed using a scanning electron microscope (SEM, Hitachi High-Tech Corporation, Model: FE-SEM SU8220) (observation field: 19 μm × 25 μm, magnification: 5000x). Subsequently, the ratio (y / x) of the length (x) in the thickness direction to the length (y) in the width direction (TD) of the polypropylene resin layer (Yt) was measured using the captured cross-sectional images. Furthermore, the number of polypropylene resin layers (Yt) with a ratio (y / x) of 17 or more in the observation field (layers / μm) was determined. 2 ) was sought.

[0095] (5) Apparent occupancy of polypropylene resin (Y) in film by FIB (focused ion beam)-SEM For each sample, 200 XZ layer (cross-sectional) data images were acquired consecutively at approximately 300 μm intervals, as shown in Figure 7. The XY images (viewed from directly above) were reconstructed from the above data using ImageJ, and the polypropylene resin (Y) region within the film surface was calculated using WinRooF2018 software. FIB-SEM measurement conditions Equipment: Thermo Fisher Scientific, Helios G4 UX Acceleration voltage: FIB 30kV, SEM 3kV Observed image: Backscattered electron image Observation field: 50 μm × 7 μm Display software: Thermo Fisher Scientific, Avizo Image analysis software: ImageJ

[0096] (6) Dielectric loss tangent (tanδ) A sample, in which aluminum was pre-deposited in a circular pattern on both sides of a sample film, was placed on the apparatus (HP (HEWLETT PACKARD) model: 4284A), electrodes were brought into contact from above and below, and the tanδ was measured when the current frequency was set to 1 kHz. Specifically, when an AC voltage is applied to a capacitor, power loss occurs. Let δ be the loss angle at this time, and tanδ be the dielectric loss tangent. A smaller value of tanδ indicates a better capacitor.

[0097] The raw materials for the polyester film in each example and comparative example are as follows: [Polyester film] (a) Polyester resin (X1): Polyethylene terephthalate homopolymer (polycondensation catalyst; antimony) with an intrinsic viscosity of 0.63. (b) Polyester resin (X2): Polyethylene terephthalate homopolymer (polycondensation catalyst; antimony) containing 0.5% by mass of silica particles with an average particle size of 0.8 μm and having an intrinsic viscosity of 0.65. (c) Polypropylene resin (Y): Melting point 163℃, MFR = 7.5 (Sumitomo Chemical Co., Ltd.: FLX80E4) (d) Resin having an acid anhydride structure (Z1): Maleic anhydride modified polypropylene resin (Admer QE800, melt flow rate; 9.1 g / 10 min, acid value; 4.3, manufactured by Mitsui Chemicals, Inc.) (e) Resin having an acid anhydride structure (Z2): Maleic anhydride modified polystyrene resin (ToughTec M1943, Meltflowrate; 8.0g / 10min, manufactured by Asahi Kasei Corporation)

[0098] [Example 1] A mixed raw material consisting of polyester X1, polypropylene Y, and compatibilizer Z1 in proportions of 87.5% by mass, 10% by mass, and 2.5% by mass, was supplied to an extruder. After melting at 285°C, it was extruded onto a cooling roll set to 25°C and cooled and solidified to obtain an unstretched sheet. Next, using the difference in roll peripheral speed, the film was stretched 3.2 times in the longitudinal direction (MD) at a film temperature of 85°C. This longitudinally stretched film was guided to a tenter and stretched 4.9 times in the transverse direction (TD) at 100°C. After heat treatment at 220°C, it was relaxed by 0.5% in the transverse direction to obtain a polyester film with a thickness of 6.9 μm. Figure 1 shows a cross-sectional photograph (magnification: 5000x) of the polyester film (sample film) obtained in Example 1. Table 1 shows the physical properties measured using the above method.

[0099] [Examples 2-3, Comparative Example 1] A polyester film was obtained in the same manner as in Example 1, except that the conditions were changed as shown in Table 1. Figures 2 and 3 show cross-sectional photographs (magnification: 5000x) of the polyester films (sample films) obtained in Examples 2 and 3, respectively. Furthermore, the physical properties measured by the above method for Examples 2-3 and Comparative Example 1 are shown in Tables 1 and 2.

[0100] [Table 1]

[0101] [Table 2]

[0102] Examples 1 to 3, which used a compatibilizer with a specific structure, showed good compatibility with Comparative Example 1 despite having a higher polypropylene content, and the film cross-sectional structure was found to have a flattened layered structure. Moreover, the layered structure originated from the polypropylene resin (Y) and was observed in large numbers within the observation field of view. In relation to electrical properties (dielectric loss tangent), the presence of more layers in the film thickness direction tended to result in better electrical properties. Furthermore, although the mechanism for forming the aforementioned layered structure is unknown, it is presumed that when stretching a film having a thickness in the ultrathin region (0.5 to 12 μm), particularly when the stretching ratio in the transverse direction is 4.5 times or more, even stronger forces are applied from above and below in the thickness direction of the film. The combined effect of the forces applied from above and below in the thickness direction of the film, along with the compatibilizer, further improves the adhesion between the polyester resin layer (Xt) and the polypropylene resin layer (Yt). As a result, it is presumed that, for example, by pushing out the air present at the interface between the two layers, a layered structure is formed that is less prone to forming voids. Furthermore, it was found that using a compatibilizer made of a resin having an acid anhydride structure resulted in particularly good compatibility, and further improvements in electrical properties can be expected. On the other hand, the apparent occupancy rate of polypropylene resin (Y) in a film obtained by FIB (focused ion beam)-SEM is an indicator that shows the degree of dispersion, representing how well the polypropylene resin (Y) is dispersed throughout the film in a three-dimensional manner. The inventors investigated the hypothesis that even if the cross-sectional structure (XZ plane) is at the same level, the dispersibility of polypropylene resin (Y) may differ in depth (XY plane). From the results in Figures 4 to 6 (the black areas represent polypropylene resin (Y)), it was found that there is a correlation between the apparent occupancy rate of polypropylene resin (Y) and its electrical properties (dielectric loss tangent), and that the higher the apparent occupancy rate, the better the electrical properties (dielectric loss tangent).

Claims

1. A polyester film comprising a polyester resin (X), a polypropylene resin (Y), and a compatibilizer (Z), wherein the cross-sectional structure of the film has a layered structure and comprises a polypropylene resin layer (Yt) in which the ratio (y / x) of the length in the thickness direction (x) to the length in the width direction (TD) (y) is 17 or more. The compatibilizer (Z) is a resin having an acid anhydride structure [-C(=O)-O-C(=O)-], and is present in 1 to 20 parts by mass per 100 parts by mass of the polyester resin (X). The polypropylene resin layer (Yt) has 0.08 particles / μm ２ A polyester film having the above-mentioned ratio (y / x) divided by the film thickness (t (unit: μm)) which is 1.5 or more, the film thickness being 0.5 to 10.0 μm, and the stretching ratio in the width direction (TD) in biaxial stretching being 4.5 times or more.

2. The polyester film according to claim 1, wherein the polypropylene resin layer (Yt) has a continuous structure from edge to edge in the width direction (TD) of an observation field of view of 19 μm × 25 μm.

3. The polyester film according to claim 1 or 2, having a plurality of the aforementioned polypropylene resin layers (Yt), wherein a polyester resin layer (Xt) is interposed between adjacent polypropylene resin layers (Yt) in the thickness direction.

4. A polyester film according to any one of claims 1 to 3, comprising 1 to 30 parts by mass of polypropylene resin (Y) per 100 parts by mass of polyester resin (X).

5. The polyester film according to any one of claims 1 to 4, wherein the polycondensation catalyst of the polyester resin (X) is Ti-based.

6. The polyester film according to any one of claims 1 to 5, wherein the polyester resin (X) is at least one selected from polyethylene terephthalate and polyethylene-2,6-naphthalate.

7. The polyester film according to any one of claims 1 to 6, wherein a cured resin layer is provided on at least one surface of the polyester film.

8. A polyester film according to any one of claims 1 to 7, wherein the dielectric loss tangent (tanδ) at 1 kHz is 0.54 or less.

9. A metal laminated film having a metal layer provided on at least one side of the polyester film according to any one of claims 1 to 8.

10. A polyester film according to any one of claims 1 to 8, for use in capacitors.

11. A metal laminated film according to claim 9, for use in capacitors.

12. The polyester film according to claim 10, for use in a capacitor mounted on an automobile.

13. A metal laminated film according to claim 11, for use in a capacitor mounted in an automobile.

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